LINES OF MAGNETIC DECLINATION IN THE ATLANTIC. 
195 
Table VH. — Disturbance of the Compass in H.M.S. Rattlesnake. 
Ship’s head. 
Disturbance 
towards the 
west. 
Ship’s head. 
Disturbance 
towards the 
west. 
Ship’s head. 
Disturbance 
towards the 
west. 
Ship’s head. 
Disturbance 
towards the 
west. 
N. 
-0 50 
W. 
+ 1° 30 
S- 
+ 0 30 
E. 
— 3 20 
X.N.W. 
+ 2 10 
vv.s.vv. 
+ 0 40 
S.S.E. 
— 1 50 
E.N.E. 
-4 30 
N.W. 
+ 3 40 
s.w. 
+ 1 50 
S.E. 
— 2 30 
N.E. 
— 3 50 
W.N.W. 
+ 2 33* 
s.s.w. 
+ 1 IG* 
E.S.E. 
— 2 35* 
N.N.E. 
— 2 20 
The observations at Portsmouth g-ive B= + ‘0565 with a dip (approximate) of 
+ 68° 40' : by their repetition at Port Jaekson in the September following B was 
found to have changed to — '0305, with a dip (approximate) of —62° 48'. The 
magnetism of the iron in the Rattlesnake was therefore, for the most part at least, of 
the nature of induced magnetism ; but the value of B in this case, as in others where 
the ship had materially ehanged her geographical position, was in arrear on her 
arrival at Port Jackson of the change which had taken place in the terrestrial dip ; 
it eorresponded to a dip of — 54° 5' instead of — 62° 48'. Being only concerned at 
present with the observations as far as the Cape of Good Hope, I have taken the 
V^B^+C^=+'056 at Portsmouth, which is its value derived from the observations 
at that port ; and having no materials from which the subsequent variation of this 
term might be more correctly computed, I have assumed it to have varied as the 
tangent of the dip, which is no doubt approximately true. 
The results of the observations which have been thus severally described are con- 
tained in the general table No. XII. at the close of this memoir, arranged in zones of 
latitude, and in the order of their respective longitudes. This table also contains the 
correction of each result to the mean epoch of January 1840. The original manu- 
scripts of the unpublished portion of the observations from w+ich these results are 
derived, together with tabular abstracts containing the details of the corrections 
applied for the ship’s iron, will be deposited in the Hydrographic Office. 
The subjoined Table, No. VHI., contains the particulars of the groups into which 
the results have been formed. The mean declinations at the points of intersection, 
shown in the final column of the table, constitute the elements from whieh the de- 
clination lines in the map are derived ; the lines having been drawn in accordance 
with them, with only such slight deviations as were indispensable to preserve an 
interconformity between the lines in a few instances where it was obvious that the 
elements themselves were slightly discordant with each other. 
* Interpolated. 
